The present invention relates to a photomask forming method and heat treatment equipment for forming a photomask where a resist pattern is formed on a photomask substrate.
When forming a resist pattern on a photomask substrate, an electron beam (referred to as EB hereinafter) resist has conventionally been used widely. This EB resist is not the so-called chemical amplification type resist but a polymeric material for effecting development by molecular weight difference through EB application. The thermophysical property of a positive type resist for removing the EB-applied portion by the development selectivity (molecular weight difference) will be described below.
When PMMA (polymethyl methacrylate) which is a polymeric material of a hydrocarbon system is used as the EB resist, in a process of a baking (referred to as pre-baking hereinafter) of heat treatment after the application of the resist, there is observed a close relation between a prebaking temperature and a resist sensitivity as shown in FIG. 4. In FIG. 4, the horizontal axis represents the pre-baking temperature in an arbitrary scale, while the vertical axis represents the resist sensitivity in an arbitrary scale. In FIG. 4, the resist sensitivity reduces as a position in relation to the vertical axis ascends. In a temperature range "a" exceeding a glass transition point Tg of the EB resist, a portion (e.g., a double-bond portion on the side chain) having a weak bonding strength of the constituent elements in the resist is once broken in the pre-baking stage. The broken portion is recombined in its easily stabilized state (a slackly re-cross-linked state) into a polymer in a cooling stage after the pre-baking. This increases a cross-linking ratio in accordance with the increase in the pre-baking temperature, so that the EB resist is gradually reduced in sensitivity. In a temperature range "b" higher than the temperature range "a" shown in FIG. 4, the cross-linking becomes saturated, so that the sensitivity of the EB resist enters a stable region. Furthermore, in a temperature range "c" higher than the temperature range "b" shown in FIG. 4, the EB resist material itself is decomposed by the heat treatment in the pre-baking stage to have a reduced molecular weight, and this makes the resist have a high sensitivity.
As shown in FIG. 5, there is observed a close relation between a cooling temperature and the resist sensitivity in a cooling process after the pre-baking process. In FIG. 5, the horizontal axis represents the cooling temperature, while the vertical axis represents the resist sensitivity in an arbitrary scale. As explained in connection with the temperature range "a" of FIG. 4, the cooling temperature after the pre-baking process dominates the polymeric bond state and the rate of progress of the cross-linking and operates as a very great factor for finally determining the resist sensitivity. This is because the rate of molecules that can be re-cross-linked in the cooling stage after the pre-baking process depends on a cooling rate, in particular, a rate of transition over the glass transition point. That is, the rate of progress of the cross-linking is determined by the cooling rate (cooling temperature). The cross-linking sufficiently progresses at the time of slow cooling, that is, at the time of the high cooling temperature, and therefore the EB resist comes to have a low sensitivity.
As described above, the sensitivity of the EB resist that is currently applied to the mass-production of photomasks depends on the pre-baking temperature in the pre-baking process and the cooling temperature in the subsequent cooling process. To accurately control the cooling temperature, which has no specific condition of stabilization in terms of thermophysics, is indispensable for obtaining a photomask of high dimensional accuracy.
For the method of forming a photomask using the above-mentioned EB resist, a convection type oven 20 as shown in FIG. 6 and a hot-plate oven 40 as shown in FIG. 7, each of which serves as heat treatment equipment, are widely used.
As shown in FIG. 6, the convection type oven 20 includes an inner vessel 24 arranged inside an outer vessel 25 having a door 26 and a heater 28 arranged on the lower side of the inner vessel 24 and operates to heat air by the heater 28 under the inner vessel 24 and circulate the heated air through a space between the inner vessel 24 and the outer vessel 25 by means of a fan 32 and a fan 22, thereby heating a photomask blank 21 arranged roughly at the center of the inner vessel 24, thereby a photomask blank 21 arranged generally at the center of the inner vessel 24 is heated. The convection type oven 20 is provided with filters 33 and 23 on the downwind side of the fans 32 and 22, respectively. Although the convection type oven 20 has the advantage that it can perform batch processing of a plurality of photomask blanks 21, it is structurally difficult to make uniform the intra-planar temperature distribution of the photomask blanks 21 due to the heat treatment by the convection inside the oven. The cooling process after a preheating process is generally performed by natural cooling inside a clean bench or the like. Therefore, it is actually impossible to achieve adjustment to the desired resist sensitivity, and variations in temperature of the photomask blanks 21 between photomasks and inside the photomask plane are also remarkable in the cooling stage.
FIG. 7 shows a perspective view of the hot-plate oven 40 of a horizontal plate placing system that solves the aforementioned problems. The hot-plate oven 40 has a plurality of base plates 41 arranged linearly and a conveyance arm 42 provided in the vicinity of each of the base plates 41 as shown in FIG. 7. By moving the photomask blank between the base plates 41 by means of the conveyance arm 42, the pre-baking process and the cooling process are sequentially performed.
FIG. 8 shows an enlarged sectional view of the essential part of the hot-plate oven 40 for performing the preheating process and the cooling process. Teflon (a trade name, PTFE) spacers 52 and 52 are arranged at a specified interval on a base plate 51. A photomask blank 53 is arranged horizontally so that both the end portions thereof are placed on the Teflon spacers 52 and 52. Thereby, a specified interval between the base plate 51 and the photomask blank 53 is placed by means of the Teflon spacers 52 and 52. Then, the base plate 51 under the photomask blank 53 is heated by a heater (not shown), thereby proximity-heating the photomask blank 53. According to the photomask forming method by means of this hot-plate oven 40, heat can be uniformly applied to the inside of the plane of the photomask blank 53. By setting the temperature of the base plate 51 to or around the normal temperature by water cooling or similar means, the method can also be applied to the cooling of the photomask blank 53. Therefore, taking advantage of the physical property of the EB resist shown in FIG. 5, the resist sensitivity can also be adjusted to the desired value by setting the temperature of the base plate 51 that serves as a cooling plate.
FIG. 10 shows an intra-planar dimensional distribution of a resist pattern formed by EB lithography using a photomask blank 60 of a plate thickness of 0.09 inch, which has undergone the pre-baking process and the cooling process in this hot-plate oven 40. In FIG. 10, the size of the hatched square mark 61 represents the positive shift amount relative to an intra-planar dimensional mean value, while the size of the white square mark 62 represents the negative shift amount relative to the intra-planar dimensional mean value. By uniform heat treatment for the photomask blank 60 in the pre-baking stage and the cooling stage, a satisfactory resist pattern intra-planar dimensional distribution can be obtained.
However, as shown in FIG. 11, if the resist pattern intra-planar dimensional distribution of a photomask blank 70 is observed after subjecting the photomask blank 70 of a plate thickness of 0.25 inch to similar pre-baking process and the cooling process, then there is a significant deviation (resist sensitivity: center portion&lt;peripheral portion) in terms of the dimensional distribution of the resist pattern. This is ascribed to the resist that has come to have a high sensitivity in the peripheral portion as a consequence of a deterioration in the intra-planar temperature uniformity of the photomask blank 70 due to an increase in heat radiation from the end surface side of the photomask blank 70 because of the increased plate thickness of the photomask blank 70. It is to be noted that the size of the hatched square mark 71 represents the positive shift amount relative to the intra-planar dimensional mean value, while the size of the white square mark 72 represents the negative shift amount relative to the intra-planar dimensional mean value in FIG. 11.
The cause of the dimensional distribution as shown in FIG. 11 will be described by means of a model representing heat flow shown in FIG. 9. FIG. 9 shows the cooling process after the pre-baking, which exerts a great influence on the resist sensitivity. The heat absorbing effect of the cooling plate of the aforementioned hot-plate oven 40 can be considered to be uniform with respect to the inside of the underside surface of the photomask blank 53. However, because of the presence of heat radiation 55 from the end surface portion of the photomask blank 53, the cooling rate becomes faster in the end surface portion than in the center portion of the photomask blank 53. This phenomenon becomes significant particularly in a photomask having a great plate thickness, and this phenomenon can be confirmed by the intra-planar temperature distribution by means of a thermograph. Due to this intra-planar temperature distribution nonuniformity of the photomask blank in the cooling stage, the resist located in the peripheral portion comes to have a high sensitivity relative to the center portion due to the thermophysical property of the aforementioned EB resist.
As a measure for preventing the deviation in the dimensional distribution of the photomask as described above for the purpose of uniforming the intra-planar temperature of the photomask blank through the prevention of heat radiation, there have been proposed the method of employing on the periphery of the photomask blank a frame-shaped member having the same heat conductivity (refer to the prior art reference of Japanese Patent Laid-Open Publication No. SHO 60-43655) and the method of providing a side plate for the end surface of the photomask blank (refer to the prior art reference of Japanese Patent Laid-Open Publication No. HEI 6-216020). However, the method of using the frame-shaped member having the same heat conductivity on the periphery of the photomask blank, although producing a great effect in uniforming the intra-planar temperature of the photomask blank, has the problem that an increase in size of the equipment cannot be avoided due to an increase in size of the board to be processed because of the employed frame-shaped member. The method of providing the side plate for the end surface of the photomask blank has the problem that the intra-planar temperature uniformity of the photomask blank is insufficient since the outward heat radiation from the end surface of the photomask blank cannot be sufficiently avoided particularly in the cooling stage.
The dimensional accuracy of the photomask is defined by the intra-planar dimensional accuracy and the amount of deviation of the mean value of the intra-planar dimension from a target dimension, and the amount of deviation from the target dimension can be corrected to some extent by adjusting the exposure light quantity in a photoresist process on a wafer. However, in regard to the intra-planar dimensional accuracy, the whole surface of the photomask is subjected to exposure of a single light quantity, and therefore, the dimensional deviation is transferred as it is onto the wafer. Therefore, in order to obtain high dimensional accuracy in the photoresist process on the wafer, it is most important to assure the intraplanar dimensional accuracy of the photomask.